dm era: Fix bitset memory leaks

commit 904e6b266619c2da5c58b5dce14ae30629e39645 upstream.

Deallocate the memory allocated for the in-core bitsets when destroying
the target and in error paths.

Fixes: eec40579 ("dm: add era target")
Cc: stable@vger.kernel.org # v3.15+
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Reviewed-by: Ming-Hung Tsai <mtsai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>